Patent GB-1393013-A (GB)
Title: Television display apparatus employing convergence correction
IPC: H04N3/16
Filed: 1972-05-09
Abstract: 1393013 Cathode-ray tube circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 9 May 1972 [12 May 1971 3 July 1971 2 Oct 1971] 21535/72 Heading H4T In a colour television display system employing, a multi-beam cathode-ray tube having a deflection system in which the line and frame coils each comprise two equal coil halves substantially without anisotropic astigmatism, a convergence arrangement effecting convergence along the axes and in the corners of the displayed image and a raster correction circuit correcting geometrical distortions of the image, residual convergence errors are corrected by modifying the raster correction circuit such that it also provides a sinusoidal correction current of line frequency amplitude modulated by a field frequency sawtooth wave the current flowing in one field deflection coil half in the same direction as the deflection current and in the other field deflection coil half in the opposite direction to the deflection current. This arrangement produces a magnetic quadripolar field which, in known manner, removes the residual convergence errors and the present invention is concerned with employing the existing raster correction circuits (e.g. for pincushion correction), to provide the sinusoidal correction current (for residual convergence correction), in addition to the raster correction current. Fig. 1, shows the known basic system in which line deflection coil halves 5<SP>1</SP>, 5<SP>11</SP> and field deflection coil halves 6<SP>1</SP>, 6<SP>11</SP> (of a deflection yoke substantially without anisotropic astigmatism), are supplied with deflection currents i H  and i V  from respective generators 8 (line) and 9 (field) together with a raster correction circuit 10 having a portion 10<SP>1</SP> (EAST-WEST correction), which amplitude modulates the line deflection current at field frequency and a portion 10<SP>11</SP> (NORTH-SOUTH correction), which generates a line frequency correction current iN amplitude modulated at field frequency. Additionally, a convergence correction circuit 7 supplied from generators 8 and 9 provides dynamic (and static) convergence along the vertical axis 11 (Fig. 2), the horizontal axis 12 and the sides 13<SP>1</SP>, 13<SP>11</SP>, but, as shown, residual convergence errors in the "landing points" of the Red, Green and Blue beams 4R, 4G and 4B are still present and these are corrected by producing the magnetic quadripolar field referred to above. This field may be produced by means of the line deflection coils (Fig. 3, not shown), but, since the NORTH-SOUTH correction circuit 10<SP>11</SP> is connected in series with the field deflection coil halves 6<SP>1</SP>, 6<SP>11</SP> a simpler arrangement utilizing the latter coils is preferred. Thus, as shown in Fig. 6, the circuit 10<SP>11</SP> includes a generator 18 providing a line frequency sinusoidal wave amplitude modulated by a field frequency sawtooth wave supplying the raster distortion correction current iN which flows in the same direction as the field deflection current i V  derived from the field deflection generator 9 the current iN having a cosine form due to the inductive load presented by inductance 19<SP>11</SP> and capacitor 22 which is tuned to a frequency a little below line frequency (e.g. tuned to 12À5 KHz for a 15,625 Hz line frequency), to produce a better approximation of a parabolic shape for the raster correction current i N . The correction current for the residual convergence errors is then produced by the voltage across the capacitor-resistor combination 23, 24, which has the same shape as that of generator 18, and results in the correction current i K  (see also Fig. 4), flowing in the same and in the opposite direction in coils 611 and 6<SP>1</SP> respectively to that of the combined deflection and raster correction currents i V  + iN, the generator 9 being shunted by a series resonant circuit 20, 21, tuned to line frequency so as to effectively short-circuit generator 9 for this frequency. Because the field deflection current from generator 16 is "S" corrected the correction effected may be too large at the commencement and at the end of the field scan period and to compensate for this a voltage dependent resistor VDR is shunted across one of the deflection coil halve - 6<SP>1</SP>. In a modification (Fig. 9, not shown), the raster correction circuit 10<SP>11</SP> is modified such that the correction current iN is the sum of two cosine-shaped currents one of line frequency and the other of twice line frequency in order to further improve the convergence or to allow deflection coils producing comparatively large convergence errors to be employed. The embodiment of Fig. 6, may be modified to enable the coil halves 6<SP>1</SP>, 6<SP>11</SP> to be fed in parallel from the field deflection generator (Fig. 12, not shown). Modifications of the arrangement of Fig. 9 (not shown), are described with reference to Figs. 13 and 14 (neither shown), and an improvement allowing the central zero position of the correction current i K  (Fig. 4), to be adjusted separately from adjustment of the zero of the NORTH-SOUTH raster correction waveform by the addition to either the line or field coil halves of an adjustable line frequency sinusoidal current flowing in opposite directions in the coil halves to produce a further quadripolar filled is described with reference to Fig. 16 (not shown). The NORTH-SOUTH correction circuit 18 may be of known type either active, e.g. employing a class B transistor output stage or passive, employing a transductor (Fig. 11, not shown).